The functioning of the central nervous system depends on its organization into laminar or nuclear compartments and the selectivity of afferent and efferent channels of connectivity with respect to the segregated ensembles. In development, the assembly of migratory neurons into compartments is conceived as the primary event, setting the stage for the prolonged processes of differentiation resulting in specific projection patterns between ensembles. The proposed research program in the fetal mouse seeks to elucidate the factors responsible for early lamination of Purkinje neurons and deep nuclear neurons in the cerebellum and for the establishment of a non-laminar, patch-matrix neuronal compartmentation in the striatum. The comparative studies will evaluate our working hypothesis that differential timing and gradients of histogenesis in combination with dynamic changes in the loss and deployment of radial glial are principle factors in the formation of patterns of cell compartmentation. These studies will make use of (3H)thymidine autoradiography to identify cells by birth date, light and electron microscopic immunocytochemistry with new antibody markers of the astrocyte lineage and specific neuron sets and subsets, and computer-assisted three dimensional reconstruction and quantification of emerging neuronal compartments and astroglial elements. The alternative hypothesis that neurons, once generated, are endowed with a capacity to sort themselves into compartments form a mixed state, will be evaluated in similar anatomical studies on the cerebellum of the reeler mutant mouse, which exhibits an early defect in Purkinje cell migration, and on reaggregates of dissociated striatal cells in culture, which effectively removes the normal architecture of ventricular regions and radial glial scaffolding. Finally, candidates for molecular agents which implement critical cell-cell interactions such as NCAM, the ganglioside GD3 and the HNK glycolipid antigen, will be localized in vivo by electron microscopic immunocytochemistry, and their functional role will be assessed by antibody perturbation experiments in striatal reaggregating cultures. These studies will contribute to the foundation for understanding grave defects of neuron migration associated with mental retardation in humans and for degenerative disorders of the cerebellum and basal ganglia.